186 Scientific Proceedings, Royal Dublin Society. 



The movement of the hair was made by means of a micrometer screw with 

 graduated head or disc, the circumference of the disc being divided into 

 300 divisions ; one whole turn of the micrometer screw gave a motion to the 

 hair of 6 x 10" 3 cms., and therefore one division on the disc corresponded to 

 a motion of 2 x 10~ 5 cms. By turning the micrometer screw always in the 

 one direction during the measurements — so as to avoid any back-lash on the 

 thread — it was found that the same readings for a given magnetic field could 

 be obtained, and each value of the reading taken is the mean of five distinct 

 and separate measurements. 



In order to get a definite and clearly defined mark for reading the 

 elongaiion of the wire with the microscope, a light brass frame was fixed on 

 the brass cylinder at the lower end of the wire; in this frame there were fixed 

 two very fine crossed spider lines, and when these lines were illuminated by 

 the light from a 50 c. p. electric glow lamp the point of crossing of the spider 

 lines was sharply defined. 



Section I. 



■ Longitudinal Magnetic Fields. 



The solenoid used in this part of the work was 236 cms. long and 2 cms. 

 internal diameter, and consisted of 7707 turns of wire in four layers, giving 

 an internal magnetic field of 41 c.g.s. units per ampere, the field being uniform 

 throughout the length of the coil to within 5 cms. of each end. The wire 

 under test was hung vertically in the middle of the solenoid, and its free 

 length (226 cms.) was in a uniform magnetic field throughout. The upper 

 attachment was made by means of a small three-jaw clutch which gripped 

 the wire 5 cms. inside the solenoid, and was firmly fixed to a bracket, the 

 bracket itself having been previously tested for rigidity. At the lower end 

 of the wire there was a brass cylinder about 12 cms. long and one centimetre 

 in diameter, having a three-jaw clutch at each end; one clutch gripped the 

 lower end of the wire 5 ems. inside the solenoid, and in the other was fixed a 

 parallel round steel rod. About half way down this steel rod or wire, there 

 was soldered a piece of brass of square cross-section four centimetres long 

 and 0'5 cm. on the side. This square brass piece worked smoothly in a 

 square hole cut in a thick brass plate fixed on a slate slab, and when well 

 lubricated with vaseline the friction was found to be negligible. To the lower 

 end of the steel rod — which went through a hole in the slate slab— under- 

 neath the slab there was fixed a scale-pan for carrying the load on the wire 

 under test. 



